1. Field of the Invention
The invention relates to displays particularly with respect to liquid crystal, multigap color displays. Such displays typically are of active matrix configuration.
2. Description of the Prior Art
Backlighted liquid crystal displays (LCD) utilizing twisted-nematic (TN) liquid crystal have been developed to provide flat panel displays for applications such as aircraft instrumentation, laptop and notebook computers, and the like. Such LCDs typically utilize a back electrode structure in the form of a matrix of transparent metal pixels or dot electrodes and a continuous transparent metal front electrode with the liquid crystal material sandwiched therebetween. The front electrode is often denoted as the common or counter electrode. Each pixel electrode is activated through a switch, usually implemented as a thin film transistor (TFT), which is deposited as a field effect transistor (FET). The drain electrode of each TFT is connected to, or actually forms, the pixel electrode with which it is associated. The gate electrodes of the TFTs in each row of the matrix are commonly connected to a gate bus-line for the row and the source electrodes of the TFTs in each column of the matrix are commonly connected to a source bus-line for the column. An image is created in raster fashion by sequentially scanning the gate bus rows while applying information signals to the source bus columns.
As is known, such LCDs are prone to anomalous image retention and flicker caused by parasitic capacitance between the gate and drain electrodes of the TFTs. The gate bus scanning pulses charge the parasitic capacitance to an offset DC voltage that results in image retention. In such LCDs, the cell gap between the back pixel electrode and the front common electrode for each pixel cell is usually uniform across the display. Such an LCD is denoted as a monogap display. A DC bias voltage is applied to the common electrode to compensate for the offset voltage so as to reduce the image retention and flicker anomaly. In other words, the DC bias voltage is applied to the counter electrode as compensation to minimize the net DC voltage across the pixel electrodes.
Color capability is imparted to the LCD by grouping the pixels into color groups such as triads, quads, and the like, and providing color filters at the front surface of the LCD to intercept the light transmitted through the respective pixels. For example, triads with primary color RED, GREEN and BLUE filters are often utilized. By appropriate video control of the gate and source buses various colors are generated.
Color LCDs are usually manufactured with a uniform cell gap for all color dots across the display active area. Because of the properties of TN color monogap LCDs, a different level of off-state luminance occurs for each of the color dots. This phenomenon results in undesirably high levels of background luminance. The condition is exacerbated when the display is viewed from varying angles since each color dot changes luminance with viewing angle at different rates, some increasing and some decreasing. The result is objectionably different chromaticities of background color for various angles of view. Additionally, this aspect of monogap LCD technology results in high levels of background luminance with viewing angle, producing undesirable secondary effects in viewability of display symbology.
Specifically, a RED, GREEN, BLUE (RGB) multicolor display requires an illumination source having strong spectral emissions at 435 nm, 545 nm, and 610 nm. It is impossible to obtain minimum background (off) transmission for all three wavelengths utilizing a display configured with a single cell gap. In such a monogap display, emissions from at least two of the three wavelengths leak through the display background resulting in increased background luminance. This, in turn, results in reduced contrast and a chromatic background.
The solution to the problem of background luminance and chromaticity is to use a multigap display with different cell gaps for individual wavelengths. In other words, for each color, the liquid crystal cell is constructed such that each cell gap is set to minimize off-state cell transmission for that color.
Such a multigap display construction permits dots to be more fully extinguished, producing more saturated, stable primary colors over the viewing angle. Any chromaticity of background, including achromatic, can be obtained with the multigap technology through the selection of different color dyes for each of the primary colors, selecting the appropriate cell gap for each primary color. Once the selection is made, the resulting chromaticity remains consistent over all viewing angles. Thus, a multigap display exhibits a consistent and predictable mixture of primary colors over the viewing angles which results in unchanging chromaticity, providing, if desired, an achromatic background over all viewing angles. This is unlike the monogap display which suffers from the deficiencies discussed above.
The multigap construction is effected by utilizing various thicknesses for the primary color filters. Since the counter electrode is disposed at the rear of the filters, the appropriate differing gaps are formed with respect to the back pixel electrodes.
Notwithstanding the advantages of the multigap technology in eliminating the background luminosity and chromaticity problem of the monogap display, the multigap construction exacerbates the image retention and flicker problems. In a multigap display, the primary color pixels have different cell gaps to maximize the off-state optical performance as discussed above. The differing gaps result in differing capacitance values for the primary color pixels. This construction makes it impossible to compensate for the gate-drain capacitance/gate voltage induced DC voltage with a single DC bias voltage resulting in image retention and flicker. There is no single counter electrode voltage capable of compensating for the different induced DC voltages on the primary pixels. For example, in an RGB triad display, if a bias voltage is selected to minimize GREEN DC, increased DC is generated in the BLUE and RED pixels.